This invention relates to a PWM (pulse width modulation) amplifier.
A PWM amplifier, also referred to as a class-D amplifier, may be used, for example, for amplifying audio signals.
A PWM amplifier of this type is known from U.S. Pat. No. 4,504,793.
This patent describes such an amplifier which comprises an integrator and a comparator.
A drawback of this circuit is that, due to stability requirements, the maximally achievable loop gain is rather low, for example, 40 dB at 1 kHz input signal and 330 kHz switching frequency. Consequently, the distortion numbers and the PSRR (power supply reaction ratio) are not very satisfactory. Moreover, the oscillator is directly coupled to the input of the circuit and the noise in the audio band produced by this oscillator will be added to the input signal. To achieve a large dynamic range, an oscillator producing minimal noise must be used. This configuration is used in, for example, the Sony TA-N88 audio power amplifier.
A further drawback of this circuit is that the switching frequency depends on the input signal.
It is, inter alia, an object of the invention to provide a PWM amplifier which does not have the above-mentioned drawbacks. A first aspect of the invention provides a PWM amplifier as defined in claim 1.
A PWM amplifier in which the distortion of the output stage as well as the noise contribution of the oscillator are reduced is realized by applying a higher-order integration in the feedback loop.
In accordance with a second aspect of the invention, a PWM amplifier has the characteristic features defined in claim 2.
A third, fourth and fifth aspect of the invention provides a receiver, a multimedia apparatus and an information reading apparatus as defined in claims 3, 4 and 5, respectively.